"Thuban" and "Zosma" Phenom II processors support AMD's Turbo Core overclocking performance-boost technology. When not all cores are needed, the processor will automatically overclock up to half of the cores by up to 500 MHz, leaving the other half idle. In turn, when the application demands more than half of the cores, the processor will run on standard clock rate and with all cores enabled.[10]

This means that 4.4GHz can only be reached if you're using only 2 cores.

The other situation is that, at least with Intel processors, the Linux kernel is not very able to figure out the actual speed of the processor. I've seen this several times on machines that have serious overclock in them, but the kernel only sees the maximum stock speed.